1. Field of the Invention
The present invention relates to a multihop relay wireless communication system. More particularly, the present invention relates to an apparatus and a method for selectively using a relay mode based on channel states associated with relay stations in the wireless communication system.
2. Description of the Related Art
The fourth generation (4G) mobile communication system utilizes cells with a relatively small radius in order to accommodate high-speed communications and a greater amount of communications traffic. It has been determined that it is unfeasible to design the 4G communication system in a centralized manner. Hence, the 4G communication system will be implemented in a distributed manner, thereby enabling it to actively cope with an environmental change such as the addition of a new base station. That is, the 4G communication system requires a self-configurable wireless network capable of configuring a wireless network autonomously or distributively.
In implementing a self-configurable wireless network, the 4G communication system adopts a technique employed by ad-hoc networks. In particular, the 4G communication system realizes the self-configurable wireless network by applying a multihop relay mode of an ad-hoc network to a wireless network that includes a stationary base station.
Communicating through a direct link between the fixed base station and a mobile station, a typical wireless communication system can establish a radio communication link with a high degree of reliability between the mobile station and the base station. Yet, the wireless network configuration of the wireless communication system is not easily adaptable because of the fixed position of the base station. As a result, the wireless communication system is unable to provide effective services in a wireless environment when there is a significant change in the distribution of traffic or in the required amount of traffic.
To address those shortcomings, the wireless communication system can employ a relay service which delivers data via multiple hops using several mobile stations or several relay stations. The relay wireless communication system is able to reconfigure the network in reaction to the change in the communication environment and is able to operate the entire wireless network more efficiently.
The wireless communication system can provide a radio channel with a better channel state to the mobile station by installing a relay station between the base station and the mobile station and establishing a multihop relay path via the relay station. In a cell boundary area with a poor channel state from the base station, the wireless communication system can offer a high-speed data channel and the extended cell service coverage area by using the multihop relay mode via the relay station to provide service.
FIG. 1 depicts a conventional multihop relay wireless communication system.
Mobile Stations (MSs) 140 through 170 in the multihop relay wireless communication system of FIG. 1 can use a broadband wireless access service from a Base Station (BS) 100 and Relay Stations 110, 120 and 130.
The BS 100 communications with the MSs 140 and 150 in its service coverage 101 through direct links L1. In doing so, the BS 100 can increase a transmission capacity with the MS 150, which travels in the cell boundary area or in a shadow region that is severely shielded by buildings and thus suffers from a bad channel state, through a relay link L2 and R1 using the RS2 130.
The BS 100 communicates with the MSs 160 and 170 outside its service coverage 101 through relay links L3 and RSL1 via the RS1 110. The BS 100 can extend its cell coverage area by providing the communication links to the MSs 160 and 170 outside the service coverage area using the RS1 110. In doing so, the RS1 110 can increase a transmission capacity with the MS4 170, which travels in the cell boundary area and suffers from a bad channel state, using a relay link L4, R2 and RSL1 using the RS2 120 and the RS1 110.
The wireless communication system can utilize a simple relay mode which provides the relay service to the MS via a single RS, and a cooperative relay mode which services the MS via a plurality of RSs.
Compared with the cooperative relay mode, the simple relay mode features a lower Carrier to Interference and Noise Ratio (CINR) in signals received at the MS through the relay service. In other words, the cooperative relay mode exhibits a higher CINR in the signals received at the MS when compared to the simple relay mode.
In addition, the simple relay mode has higher band efficiency than the cooperative relay mode. In other words, the band efficiency in the cooperative relay mode deteriorates more than in the simple relay mode.
As discussed above, the simple relay mode and the cooperative relay mode of the wireless communication system have different features. Therefore, the wireless communication system requires a method for selectively using the relay mode based on the channel state between the RS and the MSs.